Large rectangle of heavy foil.
This fabulous, quite valuable sample of tantalum was most graciously donated by .George (not 007) Lazenby, who saw the slashdot discussion. I don't know the exact thickness, but it's quite stiff and solid for a foil. It didn't quite fit under the tile, so I trimmed it square and carved an area over the sample hole so it could serve as a sub-lid underneath the wooden lid.
George got this and the other samples he donated at a surplus auction of his highschool. It seems he was the only one there who appreciated the erbium foil. This sample and the others he sent had been donated to the school by a former student who worked at NIST (National Institute for Standards and Testing), where they had been used as x-ray calibration standards. This means they are probably very pure.
In fact, analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) indicates that it is basically pure tantalum within the detection limits of the instrument.Source:George (not 007) LazenbyContributor:George (not 007) LazenbyAcquired:9 June, 2002Price: DonatedSize: 3.5"Purity: >99.9%